This invention relates to a steel sheet having a zinc or zinc-alloy plated layer having an improved welding continuity during spot welding.
This invention also relates to a method for producing a surface-treated steel sheet having such improved weldability, as well as a method for producing a surface-treated steel sheet having improved plating properties by which steel sheets such as high tensile strength steel sheets, which are difficult to deposit a plated layer by conventional methods, may be plated without causing any plating failure resulting in bare spot or uncovered area.
Zinc and zinc-alloy plated steel sheets are often used for body of an automobile to prevent rust generation. However, during spot welding of the steel sheets in the assembly of the car body, the zinc or zinc alloy plated layer melts at the interface between the plated layer and copper-based electrode, and the molten metal deposits on the electrode. Consequently, the area through which weldable current passes will be smaller than the case of cold rolled steel sheets without any zinc or zinc-alloy layer. The molten zinc or zinc alloy also erodes the copper-based electrode to damage the electrode, resulting in poor welding continuity. Productivity is thus reduced since change and dressing of the electrode are frequently required.
Various approaches are disclosed to improve the weldability of the zinc or zinc-alloy plated steel sheets. Japanese Patent Application Kokai Nos. 55-110183 and 60-63394 disclose formation of an oxide film such as Al.sub.2 O.sub.3 on the surface of the zinc or zinc-alloy layer to utilize the high melting point and high electric resistance of the oxide for the improvement of weldability. The oxide film also prevents the electrode from contacting with the zinc or zinc alloy, and prevents the melt loss of the electrode thereby extending the life of the electrode. Japanese Patent Application Kokai No. 02-04983 discloses a heat treatment of the zinc or zinc-alloy plated steel sheet to form an oxide film mainly comprising ZnO on the surface of the plated steel sheet to improve the weldability.
These approaches wherein an oxide film is formed on the zinc or zinc-alloy plated steel sheet have so far failed to achieve sufficient results in an industrial scale. These approaches were also disadvantageous in productivity in the subsequent steps including phosphate treatment and coating, as well as the quality of the resulting product.
Zinc or zinc-alloy plated steel sheets are often used for the body of an automobile as mentioned above, and also, for the exterior member of home electric appliances. Among the zinc or zinc-alloy plated steel sheets, galvanized steel sheets, especially galvannealed steel sheets, are enjoying a rapidly increasing demand for automobile rust-proof steel sheets owing to their excellent coating adhesion and corrosion resistance after coating.
Nowadays, demand for the galvanized steel sheets have changed with the drift of the trend of society. For example, improvement in fuel economy of the automobile is required in consideration of environmental issues, especially for the reduction of carbon dioxide generation. One of the most effective solutions is weight reduction of the car body. In other words, there is an increasing demand for high strength galvannealed steel sheets for an automobile, whose thickness may be reduced without detracting from various physical properties including workability, weldability and corrosion resistance. To meet such a demand, there is required an addition of one or more alloying elements selected from phosphorus, silicon, manganese and chromium which contribute to an improvement in the strength of the steel sheet to an extra low carbon steel sheet having at least one element selected from titanium, niobium and boron added thereto without detracting from the workability of the steel sheet.
The alloying elements such as phosphorus, silicon, and chromium are easily oxidized and difficult to reduce. Therefore, in the annealing step of a continuous galvanizing line, for example, Sendzimir line, these alloying elements frequently form stable oxides on the surface of the steel sheet, and also the alloying elements often segregate underneath the thus formed oxides. These oxides will not be fully reduced even when the steel sheets are annealed in a reducing atmosphere, and the oxides which inconsistently remained will inhibit wetting of the steel sheet surface in the galvanizing after the annealing and cooling of the steel sheet, resulting in a plating failure such as bare spots and, in more serious case, uncovered areas. The inconsistently remained oxide will lead to a significant reduction of adhesion of the plated layer even when no plating failure is induced. In the galvannealing, the inconsistently remained oxides will result in an inconsistent alloying of the plated layer, resulting in uneven plated surface. In more serious cases, visually recognizable unevenness commonly referred to as white or black streak will appear on the surface.
Various approaches have been proposed to galvanize or galvanneal these steel sheets, which are difficult to plate, without causing any plating failure, and without causing inconsistent alloying resulting in uneveness or streaking. These approaches employ various pretreatments of the steel sheet surface. Japanese Patent Application Kokai No. 55-43629 discloses deposition of a copper layer on the steel sheet, and Japanese Patent Application Kokai No. 55-131165 discloses deposition of a nickel layer on the steel sheet. Japanese Patent Application Kokai Nos. 57-70268 and 57-79160 disclose deposition of an iron layer on the steel sheet.
These approaches, however, suffer from various problems in their practical uses. When a copper layer is plated on the steel sheet, copper will dissolve into the zinc plating bath to contaminate the zinc bath. When a nickel layer is plated on the steel sheet, nickel will also dissolve into the zinc plating bath to contaminate the zinc bath. Furthermore, in galvannealing, nickel will excessively promote the alloying reaction, and in some extreme cases, alloying may start as early as in the galvanizing step. Consequently, control of the alloying will be quite difficult. In contrast to the copper and nickel plated layers, an iron layer little suffer from the contamination of the zinc plating bath. Iron layer containing iron alone, however, is far from being effective.